An electric heating system typically includes a plurality of electrically resistive heating elements, which give off heat in response to the passage of electric current therethrough, and a blower for circulating air between the heating system and an enclosure. According to prior practice, a demand for heat signal from an indoor thermostat or the like activates the heating elements and air moving across the heating elements carries the heat by convection to the enclosure to satisfy the demand for heat.
Typically, an electric heating system also includes a plurality of electrically operable sequencers for sequentially activating the heating elements in response to a demand for heat. For example, a one minute delay between successive activations may be built in. Sequential activation of the heating elements inhibits unwanted power surges, which may occur if all of the elements are activated at the same time. Once the demand for heat signal is received, however, all of the heating elements are activated, albeit sequentially, until the demand for heat is satisfied. The heating elements are not separately controllable.
Separate control of the heating elements is desirable in order to activate only the number of heating elements required to meet the demand for heat, thereby reducing energy consumption and cost. Further, when the electric heating system is used in conjunction with a heat pump, the heating system is typically used as a backup because operation of the heat pump is usually more efficient. Therefore, only the heat pump is used to satisfy the initial demand for heat, which is referred to as the first stage heat demand. If the indoor temperature falls below a predetermined threshold temperature, which is less than the temperature corresponding to the first stage demand, a second stage demand for heat is indicated. The electric heating system is activated in response to the second stage demand. For example, an indoor thermostat may be set at 69.degree. F. If the indoor temperature drops below 69.degree. F., the thermostat sends a signal indicating a demand for first stage heating and the heat pump is activated. If the indoor temperature continues to fall and reaches a lower temperature (e.g., 65.degree. F.), the thermostat will signal a demand for second stage heating, which activates the electric heating system. The electric heating system is used only when the heat pump is unable to maintain the desired indoor temperature.
The electric heating system is used as the primary heat source when the heat pump is not able to function efficiently. For example, when the outside air temperature drops below a minimum temperature (e.g., 10.degree. F.) the heat pump may not be operable due to a low ambient temperature condition. In that case, the electric heating system is used as the primary source of heat.
Another problem associated with prior art heating systems is the problem of the blower cycling on and off when the indoor temperature is at or near the desired temperature setting, which results in excessive energy consumption and blower performance degradation. One attempt to solve this problem has involved building in a fixed time delay so that the blower continues to operate for a period of time equal to the built-in time delay after a "blower off" condition is indicated. One problem with this approach is that the optimum time between blower cycles varies among individual heating systems. Therefore, a fixed time delay is selected as a compromise, which may not be optimal for a particular system.